Monostable multivibrator



p 25, 1956 R. E. GRAHAM MONOSTABLE MULTIVIBRATOR 2 Sheets-Sheet 1 FiledDec. 23, 1952 POTENTIAL SIGNAL souecs SYNCl-IRON/Z/NG INVENTOR R. 1:.GRAHAM BY A1 4 & 4/-

A TTORNEV United States Patent 'Ofiice 2,764,677 Patented Sept. 25, 1956MONOSTABLE MULTIVIBRATOR Robert E. Graham, Chatham Township, MorrisCounty, N. 1., assignor to Bell Telephone Laboratories, Incorporated,New York, N. Y., a corporation of New York Application December 23,1952, Serial No. 327,611

4 Claims. (Cl. 250-27) This invention relates to signal generatorsystems and more specifically to such systems of the multivibrator typewhich operate only in response to a triggering pulse.

In some types of electronic circuits, a signal generating system isfrequently required that will, in response to an external timing ortriggering pulse, provide a non-sinusoidal output pulse for a briefperiod. One such system, known as a monostable or one-shotmultivibrator, includes a pair of coupled electron discharge tubes, oneof which conducts only during the stable or off period of multivibratorand the other of which conducts only during its unstable or on period.The system provides an output pulse during the on period. The beginningof the on period is determined by the external triggering pulse and theduration of this period (and hence, the duration of the output pulse) isdetermined by the time constant of the coupling network connecting thetwo tubes.

It is an object of this invention to simplify and make more compactmonostable multivibrators capable of pro viding output pulses whoseduration-may be of the order of microseconds or less and which can bevaried over a wide range.

This invention contemplates the use of a pair of crosscoupled electrondischarge devices or vacuum tubes. The coupling is (1) between the anodeof one tube (called the first tube) and the grid of the other (second)tube and (2) between the cathodes of the two tubes. The cathode couplingnetwork includes a capacitor connected between the cathodes which actsas a low impedance connection during the initial stage of themultivibrator on period to produce regeneration. During the remainder ofthis period, the cathode network serves as a timing circuit whichcontrols the multivibrator on period. Unlike other monostablemultivibrators of this type, the multivibrator circuit requires but twopoints of fixed potential throughout.

Additionally, and also unlike previously known monostable multivibratorsof this general type, the grid of the second tube is biased by aninternal impedance arrangement in the circuit whereby the grid ismaintained at a constant potential with respect to its correspondingcathode during the off period irrespective of any fluctuations in thepotential levels of the coupled cathodes. The biasing means provides animportant advantage by insuring that the multivibrator will not betriggered into an on period by inopportune fluctuations of the voltagelevel in the cathode circuit, and further insures that thegrid-to-cathode bias will always be maintained within such limits as topermit the small triggering pulse reaching the grid of the second tubeto trigger it into its conductive state.

Other advantages will be apparent and the invention will be more clearlyunderstood from the following descrip tion taken in connection with theaccompanying drawings forming a part thereof, in which:

Fig. 1 is a circuit diagram of a multivibrator in accordance with theinvention; and

Figs. 2A to 2F, inclusive, show voltage wave forms appearing on variouselements of the multivibrator circuit.

With specific reference to the drawing, there is shown in Fig. 1, asanexemplary embodiment of the invention, a multivibrator system whichincludes a twin triode vacuum tube VI comprising a first triode sectionincluding an anode or plate element 11, a grid element 12 and a cathodeelement 13, a second triode section comprising an anode or plate element14, a grid element 15 and a cathode element 16, a heater element commonto both cathode elements, and a shield. The anode 11 is connected to thepositive terminal of a potential source 17 through series-connected loadresistor 19 and resistor 18. A condenser 20 connected between thejunction of resistors 18 and 19 and ground potential provides fordecoupling the plate from the potential source. Anode 14 is connected tothe positive terminal of source 17 through resistor 21. The anode 14 isconnected to ground through a condenser 29.

Current is supplied to grid 12 from the positive terminal source 17 bythe grid resistor 25 which is connected to the grid through theanti-sing resistor 24. Through the use of a potential dividingarrangement comprising the series connected resistors 32 and 33, one endterminal of which is connected to the junction of resistors 24 and 25,the common terminal of which is connected to grid 15 through anti-singresistor 31 and the other end terminal of which is connected to thenegative terminal of source 17, the potential between grids 12 and 15 isheld substantially constant during the OE period. It is obvious that apotentiometer may be used instead of resistors 32 and 33. Cathode 13 isconnected to ground and to the negative terminal of source 17 throughload resistor 26, and cathode 16 is coupled to cathode 13 and resistor26 through a parallel-connected capacitor 27 and resistor 28. Anode 11is coupled to grid 15 through a series-connected capacitor 30 andanti-sing resistor 31. The external timing or triggering wavefrom thesynchronous signal source 22 is applied to grid 12 through theseries-connected coupling capacitor 23 and the resistor 24. One outputterminal 34 is connected to the anode 11 and the other terminal 35 isconnected to the negative terminal of source 17. This output terminalpair in turn supplies an output pulse to a useful output load hererepresented by 36. With reference to the mode of operation of thecircuit arrangement just described, prior to the appearance of thetriggering pulse in the circuit the multivibrator is o with the firstsection of tube VI conducting at approximately zero grid-to-cathodepotential and the second section of the tube substantially cut-off withthe cathode positive with respect to the grid. The first section of tubeV1 is maintained at zero grid-to-cathode potential by an appropriatechoice of values for the potential source 17 and for the resistors 24,25 and 26. Values for the resistors 18 and 19 are chosen in light of thevalue already given to, resistor 26, so that the anode dissipation ofthe first section will not be exceeded. -Further, the size of resistor19 will be determined by the magnitude of the output pulse desired. Thesecond triode section of V1 is held substantially at cut-oii by biasingmeans comprising resistors 32 and 33, and the cathode resistor 23.

When a negative timing or triggering pulse shown in Fig. 2A is appliedfrom source 22 to the grid 12, the grid voltage drops as shown in Fig.2B and starts cutting ofi the first section, thereby causing anamplified positive pulse shown in Fig. 2C to appear on the anode 11. Aregenerative cycle is then initiated by two actions that take place inthe circuit. The first takes place instantaneously with the initialnegative swing of grid 12 in the first triode section which causes thevoltage on cathode. 13 to fall by cathode follower action. This suddendrop in voltage is communicated to the cathode 16 of the second sectionwhich causes the voltage on this "cathode to also drop. At the sameinstant, the second (and most important) action takes place at grid ofthe second section. The positive pulse on the anode 11 is impressed ongrid 15 "through coupling condenser 30 and resistor 31, thereby causingthe voltage on grid 15 to rise as shown in Fig. 2D. The combined efiec'tof these simultaneous actions on the .grid and cathode of the secondsection is to suddenly change the grid-to-cathode bias from a cut-offlevel to a conducting level. Current begins to flow through the secondseeti'onof the tube and, because the plate'voltage of the tube'is heldconstant by the plate decoupling condenser 29 during the timing waveinterval, a cathode follower action takes place and the voltage oncathode M'rises as shown in Fig. 2E to approximately'the voltage on grid15.

The rise in cathode voltage in the second section is communicated tocathode 13, as shown in Fig. 2F, through condenser 27, thereby tendingto further reducethe current in the first triode section, increasing themagnitude ofthe positivepulse on anode '11, and further raising thevoltage on grid 15. The gain around this regenerative loop substantiallyexceeds unity so that the action proceeds with great rapidity until thefirst triode section is cut off by having its cathode driven highlypositive with respect to its grid. At the same time the second triodesection becomes conducting withits grid-to-cathode potential atsubstantially zero.

With the triggering action completed, the circuit is in temporaryequilibrium, the first triode section being cut oil, the second sectionbeing conducting and a positive output pulse being supplied at theterminals 34 and 35. It becomes apparent that this temporary equilibriumcondition exists only so long as the current through condenser 27 andresistor 26 maintains the cathode 13 positive with respect to the grid12 in an amount exceeding the cut-off potential of thetube. Immediatelyafter the triggering action is completed, the condenser 27 begins tocharge and the potential of cathode 13 consequently begins to falltowards a voltage E1 established by the voltage divider comprisingresistors -26 and 28 as shown in Fig. 2F. Inasmuchas the internalimpedance of cathode 16 is very small compared to the values ofresistors 26 and 28, the rate of fall is determined by the parallelvalue of these two resistors and the cathode coupling capacitor 27.

When the voltage on cathode 13 of the first section has fallen to avalue that permits the first triode section to conduct again, thetriggering action is repeated in reverse. A negative pulse appears onthe anode 11 of the first section which is communicated to the grid 15of the second section thereby lowering its potential with respect tocathode 16 and tending to drive this section beyond cutoif. Cathodefollower action lowers the voltage on cathode 16, which through couplingcapacitor 27 further lowers the voltage on cathode 13 of the firstsection. This, of course, places the first section in a more stableoperating condition and further tends to cut oh" the second triodesection so that the circuit is returned to its stable state with greatrapidity and the positive pulse at terminals 34 and 35 is cut ofi.

It is desirable that before the multivibrator is again triggered intoits unstable state, the capacitor be discharged to its stable statepotential. The time required to do this is determined by the value ofcondenser 27 and the value of resistor 28, which resistor provides aparallel discharge path across condenser 27.

The choice of values for resistors 26 and 28 in this circuit is seen tobe restricted by several conditions. In the first instance, resistor 26,together with resistors 19 and 18, must limit the plate dissipation ofthe first triode section. Secondly, the value of resistors 26 and 28 inparallel, together with the value of condenser 27, must determine the onperiod of the multivibrator. Thirdly, the values of resistors 26 and 28which form a voltage divider must establish a voltage towards which thecapacitor 27 charges that is well below the cathode voltage required tocause the first triode section to conduct. Finally, the time constantestablished by resistor 28; and condenser 27 must be short enough topermit the multivibrator to arrive again at its stable state so that itmay be triggered when next called upon by the synchronous signal sourceto provide an on period.

The resistor 28 not only serves in the functions described above, but italso serves as one element in the biasing means for the second triodesection, which biasing action We shall now consider.

One element of the biasing means is that portion of the potentialdivider connecting the grid circuit of the first section and the gridcircuit of the second section, which potential divider comprisesresistors 32 and 33. The values of the resistors are chosen so that thecurrent in the resistors 32 and 33 is small as compared with the gridcurrent in resistor 24 during the off condition, and resistor 33 islarge compared with resistor 32. Another element in the biasing means isresistor 28 connecting the two cathodes by which any change in thepotential of either cathode causes a similar change in the potential ofthe other cathode. Any change in the cathode voltage of the firstsection affects the grid current flow through resistor 25 which causesthe grid voltage of the first section to change in magnitude anddirection according to the change on the cathode voltage. This change ingrid voltage in the first section is repro duced on the grid of thesecond section by the potential dividing arrangement thereby causing thegrid-towathode bias of this section to be substantially constant.

To better understand this biasing action, consider the condition whereinthe voltage levels of the cathodes have increased above their givenvalues during the off period. There follows then a decrease in thegrid-to-cathode potential in the first section which is at this timeconducting, and a decrease in the grid current'in this first section.Inasmuch as the resistor 25 is large compared to anti-sing resistor 24and the internal grid-to-cathode impedance of the first section, thedecrease in grid current causes the voltage drop across resistor 25 todecrease and the voltage level of the grid 12 to increase both by anamount substantially equal to the increase in the voltage level of thecathode 13. Thus, the gridto-cathode potential of the first *sectionremains substantially constant. With resistor 32 small as compared toresistor 33, the voltage level of the grid 15 of the second section iscaused to be increased by an amount similar to that of the grid 12 ofthe first section and by an amount similar to that of cathode 1650 thatthe grid-to-cathode potential of the second section remainssubstantially constant. It is clear that inthe case where the voltagelevels of the cathodes have decreased, appropriate changes in thevoltage levels of the grids will occur to maintain the grid-to-cathodepotentials constant. The advantage of this arrangement is that thesecond triode section, which is non-conductingduring this off period, isprevented from being tripped into a conducting state by fluctuations inthe voltage levels of the cathodes and similarly there is assurance thatthe grid to-cathode bias of the second section will stay relativelyconstant during this period and. permit thezpulse appearing on grid 15of the second section to trip this section into its conductive state.

In order to decouple the pulses appearingon each anode of each tubesection from the directcurrent source 17,. resistance-capacitancenetworks comprising resistor 18 and condenser 20, and resistor 21 andcondenser 29 are provided. The time constants of these networks are longcompared to the 'on period of the multivibrator.

Multivibrators have been built in-accordance with the invention havingon periods of three microseconds and one microsecond, respectively, andit is'apparent that the circuit elements may be chosen so :asto providea multivibrator having an on period that may be as brief as 0.1microsecond. While there are a number of possible circuit constants forthe various circuit elements, values used in an operable embodiment ofthe multivibrator system of Fig. 1 (for a multivibrator in which theapplied trigger pulse is minus 2 volts and of 4 microseconds duration,the output pulse is 70 volts and the on period is of three microsecondsduration) and which are reproduced below merely by way of example are:

V1 2C51 17 volts 300 18 ohins 6200 19 do 5600 20 micrornicrofarads 910021 "ohms" 4700 23 micromicrofarads 9100 24 ohms 100 25 megohms 0.56 260hms 3300 27 micromicrofarads 1000 28 oh-ms 18000 29 micromicrofarads9100 30 do 9100 31 ohms 100 32 megohms 0.51 33 d-o.. 7.5

It is understood the above described arrangement is merely illustrativeof the application of the principles of the invention. Numerous otherarrangements might be devised by those skilled in the art withoutdeparting from the spirit and scope of the invention.

What is claimed is:

1. A multivibrator adapted to be connected to an input wave sourcecomprising a first electron discharge device and a second electrondischarge device, each of said devices including an anode, a cathode anda grid, means for coupling the anode of one of said devices to the gridof the other of said devices, means including a resistor and a capacitorconnected in parallel to one another for coupling the cathodes of saiddevices, a source of potential having a negative and a positive pole,means for connecting both of said anodes to said positive pole, meansincluding a resistor for connecting the cathode of one of said devicesto said negative pole, means including a further resistor for connectingthe grid of one of said devices to said positive pole, potentialdividing means having two external terminals and an inner terminal,means for connecting one of said external terminals to the grid of oneof said devices and to the said positive potential pole, means forconnecting the other of said external terminals to said negative pole,and means for connecting said inner terminal to the grid of the other ofsaid devices to maintain said grid at a constant potential with respectto its cathode.

2. A multivibrator according to claim 1 in further combination withmeans for applying an input wave to the grid of one only of saiddevices.

3. A multivibrator according to claim 1 wherein said two poles of saidpotential source are the only points of fixed potential in themultivibrator circuit.

4. A monostable multivibrator adapted to be connected to an input wavesource comprising a first electron discharge device and a secondelsctron discharge device, each of said devices including an anode, acathode and a grid, means for coupling the anode of said first device tothe grid of second said device, means including a resistor and acapacitor connected in parallel to one another for coupling the cathodeof said second device to the cathode of said first device, a source ofpotential having a positive and a negative pole, said poles being theonly points of fixed potential in the multivibrator, means forconnecting both of said anodes to said positive pole, means including aresistor for connecting the cathode of said first device to saidnegative pole, means including a further resistor for connecting thegrid of said first device to said positive pole, means for applying aninput wave to the grid of said first device, potential dividing meanshaving two external terminals and an inner terminal, means forconnecting one of said external terminals to the grid of said firstdevice, means for connecting the other of said external terminals tosaid negative pole, and means for connecting said inner terminal to thegrid of said second device.

References Cited in the file of this patent UNITED STATES PATENTS2,403,984 K-oenig et a1 July 16, 1946 2,459,852 Summerhayes Jan. 25,1949 2,524,134 Palmer Oct. 3, 1950 2,526,000 Bliss Oct. 17, 1950

